1. Technical Field
This invention applies to ducts having utility in gas turbine engines in general, and to cooled ducts in particular.
2. Background Information
Efficiency is a primary concern in the design of any gas turbine engine. Historically, one of the principle techniques for increasing efficiency has been to increase the gas path temperatures within the engine. The increased temperatures necessitate internally cooled components, high-temperature capacity materials, etc. A duct passage downstream (or “aft”) of the turbine is typically cooled using compressor air worked to a higher pressure, but still at a lower temperature than that of the core gas flow passing through the duct passage. The higher pressure provides the energy necessary to push the air through the walls of the duct. A significant percentage of the work imparted to the air bled from the compressor, however, is lost during the cooling process. The lost work does not add to the thrust of the engine and therefore negatively affects the overall efficiency of the engine. A person of skill in the art will recognize, therefore, that there is a tension between the efficiency gained from higher core gas path temperatures and the concomitant need to cool components and the efficiency lost from bleeding air to perform that cooling. There is, accordingly, great value in minimizing the amount of cooling air required to cool the duct passage.
To provide an acceptable amount of cooling air, it is necessary to maintain the cooling air at a predetermined pressure greater than that of the core gas flow disposed within the duct passage. The pressure difference between the core gas flow and the cooling air is typically referred to as the “backflow margin”. Core gas flow within the duct passage is seldom uniform in temperature or pressure; e.g., core gas flow within certain circumferential sections of the duct passage may typically be at a higher pressure than flow within adjacent sections. The backflow margin for prior duct passages must typically be relatively large to ensure that adequate cooling is present around the circumference of the duct passage, and undesirable hot core gas inflow is avoided. What is needed, therefore, is a duct passage that promotes desirable, efficient cooling.